Update __git_flow_list_remote_features to use prefix config.

Use gitflow's feature prefix configuration option when fetching remote feature branches. This is used to determine which remote feature branches are available when completing 'feature track'.
diff --git a/git-flow-completion.bash b/git-flow-completion.bash
index e5d66ba..c76d323 100755
--- a/git-flow-completion.bash
+++ b/git-flow-completion.bash
@@ -112,7 +112,12 @@
 
 __git_flow_list_remote_features ()
 {
-	git branch -r 2> /dev/null | grep 'origin/feature/' | awk '{ sub(/^origin\/feature\//, "", $1); print }'
+	git branch -r 2> /dev/null | grep "origin/$(__git_flow_feature_prefix)" | awk '{ sub(/^origin\/$(__git_flow_feature_prefix)/, "", $1); print }'
+}
+
+__git_flow_feature_prefix ()
+{
+	git config gitflow.prefix.feature 2> /dev/null || echo "feature/"
 }
 
 __git_flow_release ()